This invention relates generally to switching amplifiers, and more particularly to limiting transient voltages in push-pull amplifiers during switching intervals.
The use of push-pull switching amplifiers in a square wave or pulse type of operation has proven to be an effective means of achieving high efficiencies in amplifiers as well as in inverters. This is particularly true when the amplifiers operate at switching rates greater than the fundamental frequency. Such higher switching rates are provided by Class S amplifiers by using some form of pulse width modulation (PWM), pulse duration modulation (PDM) or time-ratio controlled modulation. A description of high switching rate operation can be found in Principles of Inverter Circuits by B. D. Bedford and R. G. Hoft, John Wiley and Sons, Inc., 1964, pages 254-262.
A problem heretofore has been that when push-pull transformers are used in switching amplifiers, if one half of the push-pull primary is not very tightly coupled to the other half of the push-pull primary, high transient voltages can occur during the switching interval. The transient occurs during the interval that the conducting switch is turning "off" and its mate is turning "on" and also, in Class S operation as a switch turns "on" and "off" during its half of the operating cycle. The amplitude and frequency of the transient voltage is a function of a combination of three or more of the following parameters of the transformer and its associated excitation circuit:
1. The leakage inductances of the transformer. PA1 2. The distributed capacitances and other shunt capacitances which may be across some or all of the transformer windings. PA1 3. The abruptness with which the current-carrying switch element interrupts the primary current. PA1 4. The amplitude of the primary current at the instant of interruption. PA1 5. The abruptness with which the "closing" switch element changes from a very high impedance to a very low impedance. PA1 6. The voltage across the "open" switch elements at the time the "closure" starts.
In many cases it is impossible or impractical to design a tranformer with the degree of coupling in the push-pull primary that is necessary to limit voltage transients to an acceptable level.
Circuits have been proposed which protect push-pull amplifiers from excessive currents. These protective circuits, however, have been designed to handle excessive input signals rather than internally generated transients. Thus, emphasis has been on passing the excess signal from the amplifier input to its output to bypass the individual amplification elements (generally either vacuum tubes or high-power transistors). The circuits have not provided for adequate protection against internal transients since these could effect the individual elements without going through the input terminal. Further, no means has heretofore been provided to store and/or recover any of the transient voltage energy which arises from the amplifier inductors and capacitors while still protecting the amplifier.